Because you really don't know that you need them until you actually do ? Like I said bug fixes and regressions sometimes require changes in kdelibs. If we purposefully shackle ourselves to forward compatibility then such bugs and regressions, no matter how bad, cannot be fixed until the next major release. If that is acceptable for the sake of being forward compatible, then that is fine. However, I really do not see the point of it. Why would anyone want to use new version of applications with the older version of the required libraries ?